In 1423 Thomas Somerville, as Lord of Carnwath, came to London as an ambassador to treat for the release of James I of Scotland, who had been captive in England for many years.
As Somerville of that Ilk, he sat on the assize at Stirling Castle in May 1425 that condemned Murdoch Stewart, Duke of Albany.
In addition, upon the death of his uncle, Thomas Somerville, in 1412, he succeeded to the estates of Gilmerton, Drum, and Goodtrees in the Edinburgh vicinity.
He also held lands in Quothquan and Newbigging in Lanarkshire; and Broughton in Peebleshire.
[2] Cambusnethan was brought to him by his wife, Janet, daughter of Alexander Stewart of Darnley (d.1404).